How big was the terror plot?

The discovery provides another link with the men behind the 21 July attacks on London, one of whom used a similar device. Yesterday, police arrested one of the four suspected failed suicide bombers.

Officers used a Taser stun gun to knock down Yasin Hassan Omar as he was allegedly trying to escape after they raided a house in Birmingham at 4.30am. No explosives were found. Three suspected failed bombers are still being hunted and officers mounted several raids in London and made several arrests around the country.

Police also released a new picture yesterday of the unidentified Shepherd's Bush bomber, and gave details of his escape after his device failed to go off on 21 July.

The 16 bomb parts found in Luton, including one nail device and a number of other packets of explosives, were discovered by police in a rented Nissan Micra at the main railway station five days after the 7 July attack. The types of bombs are far more alarming than police had previously disclosed. They include a Molotov cocktail-style bottle bomb, packed with explosive and studded with nails. Security agencies warned after the 7 July attacks on three Tube trains and a bus that future attacks could be against nightclubs, sports stadiums or large public gatherings. The warning could have been related to the bombs in the car used by the Leeds-based terrorists.

The confidential Metro-politan Police photographs were shown yesterday on the American television network ABC News. Pictures of the carnage caused by the suicide bomber at King's Cross, in which 27 died, were also broadcast. It is the latest leak to come from the United States and has caused anger among British police and intelligence agencies.

The disclosure of a cache of different types of bomb components raises disturbing questions. Was the car a bomb store for another team of bombers who, for whatever reason, failed to collect them? Security sources have said there is no evidence yet of a "missing" team. The nail bomb poses the question of whether terrorists were planning a different form of attack. It may be that the four suicide bombers - Mohammed Sidique Khan, 30, Shahzad Tanweer, 22, Hasib Hussain, 18, and Germaine Lindsay, 19 - merely brought with them a choice of devices and dumped the ones they did not need.

The Nissan was hired by Tanweer and is believed to have been used to bring the other two Leeds-based terrorists, Hussain and Khan. It is unclear whether explosives were found in the Fiat left at the station by Lindsay.

Weapons specialists agreed the Luton discoveries suggested the terrorists may have been planning to throw the nail bomb into crowded places.

Andy Oppenheimer, an explosives specialist for Jane's Information Group, said the photographs showed "simple and deadly" bombs. "These are easy to make and are of a type which has been used by Ulster gangs and Palestinian terrorists in Israel to deadly effect," he said.

The nail bombs appear to be milk bottles packed with explosives, possibly acetone peroxide made from household chemicals, mixed with what might be military plastic explosive to stabilise it and increase its power. There are sharp, short nails or tacks stuck on the outside and the whole thing is covered in plastic wrap. "The nails are simply there to increase the destructive power of the bomb," Mr Oppenheimer said. "They can be very deadly in a confined space."

The second type of bomb part appears to be film-wrapped packages of explosives of the type almost certainly used in the rucksack bombs carried by the 7 July suicide bombers.

In Birmingham, Mr Omar, a 24-year-old Somalian, was arrested when officers raided a house in Heybarnes Road, Hay Mills. About 100 homes were evacuated as bomb squad officers moved in. One report suggested he had a rucksack which an officer threw through a window. Mr Omar, suspected of trying to blow up a Tube train near Warren Street last Thursday, was taken to Paddington Green police station in London.

In Heybarnes Road, people said men fitting the description of Mr Omar and another suspected failed bomber, Muktar Said Ibrahim, had been seen in the area on Saturday. Shortly after the arrest, three other men were arrested two miles away in Bankdale Road, Washwood Heath, Birmingham. They are being held in Birmingham. Neighbours said three Somalian men had been staying in the semi-detached property.

Police also raided houses in Finchley and Enfield, north London, where no arrests were made, and in Stockwell, south London, where three women were arrested.

At least three suspected would-be suicide bombers from the failed 21 July attacks are still on the run. They include Mr Ibrahim, 27.Detectives are investigating reports from residents in New Southgate, north London, that the day after the failed attacks some of the four returned to a flat they had used as a bomb factory.

The bombs found in Luton

NAIL BOMBS

These are the lethal nail bombs designed to "tear into flesh" found in the arsenal of the group that committed the 7 July attacks. The devices, made from bottles packed with explosives and nails, were found in a Nissan Micra rental car used by Shahzad Tanweer, one of the bombers.

The discovery, at the car park of Luton railway station five days after the blasts, is a chilling insight into what is likely to have been the next stage in the terrorist campaign - atrocities against packed crowds at targets such as nightclubs and football matches. The devices, say weapons experts, are a departure from the tactics used by the bombers so far.

The bombs are easy and cheap to make. Possibly acetone peroxide is packed into milk bottles then wrapped in clingfilm with nails and shards of metal stuck on. The devices appeared to have a fuse protruding from the top. But they were unprimed and it is unclear whether the fuse would have been used as a detonator. Usually, they are thrown into a crowd, with the range a circumference of about 10 metres.

Robert Ayers, an explosives specialist with 30 years' experience with the US Army and the CIA who has also worked with the British military, said: " These are anti-personnel weapons, like a home-made grenade, and the effect would be to tear through flesh causing terrible, lethal injuries. An explosive charge or fuse would be used for the detonation. If the bomber wanted to die, he could detonate with a switch."

Andy Oppenheimer, an explosives specialist for Jane's Information Group, said: "These are the simple and horrible type of bomb which have been used to deadly effect by gangs in Ulster and Palestine for many years."

CIRCULAR BOMBS

These devices, around five inches in diameter and around three-quarters of an inch thick, are packed parcels of explosives, tightly wrapped in clingfilm. These are likely to have been the type of bombs used in the suicide attacks on 7 July.

The bombs would have been carried in rucksacks, and possibly within containers to make a larger device which would have a devastating impacton structures as well as human bodies.

The base explosive is believed to be homemade acetone peroxide - also known as "Mother of Satan" - perhaps mixed with a military explosive to make a far more powerful detonation.

An explosives specialist, Robert Ayers, said: "One could put in as many as the rucksacks would allow and one would have to be careful not to overfill as this could make the packets burst.

"One only has to look at the pictures of the blast sites to see the effect these bombs have."

Another specialist, Andy Oppenheimer, added that the bombs, in the state they had been discovered, were incomplete: "These are unfinished bombs waiting to have detonators attached.

"I believe there was a mixture of explosives because of the destructive effect that we can see in the pictures of the damage to the Piccadilly line train."

Kim Sengupta and Terry Kirby